The invention relates to a magneto-optic disk apparatus and, more particularly, to a magneto-optic disk apparatus in which a reproduction signal is detected from a magneto-optic disk as a recording medium, manufactured by using a disk substrate having a birefringence, by using a differential detection method.
Hitherto, as a magneto-optic disk apparatus, an apparatus disclosed in JP-A-62-65255 has been known. FIG. 1 is a schematic diagram of a magneto-optic disk and a reading optical system disclosed as FIG. 1 in the above Official Gazette.
A light beam emitted from a semiconductor laser 3A as a laser device is converted into a parallel light by a collimator lens 4A and an isotropy of an intensity of the laser beam is converted into an anisotropy by a shaping prism 5A. After that, the laser beam passes through a first beam splitter 6A and its progressing direction is changed by a mirror 7A. After that, the laser beam is irradiated onto a magneto-optic disk 1A through an objective lens 8A. As shown in an example of FIG. 2, for instance, the magneto-optic disk 1A has a structure such that a magneto-optic recording medium 1b is formed on a transparent substrate 1a. The laser beam entering the magneto-optic disk 1A passes through the transparent substrate 1a and is irradiated onto the magneto-optic recording medium 1b (hereinafter, simply referred to as a recording surface 1b). The structure of the magneto-optic disk 1A using polycarbonate as a substrate as shown in FIG. 2 is disclosed in, for example, a literature of "Optical Disk Technique", published by Radio Gijutsu Co., Ltd., pages 330 etc., Jul. 20, 1992.
A reflected light from the magneto-optic disk 1A passes through the objective lens 8A and mirror 7A and is reflected by the beam splitter 6A and progresses toward a second beam splitter 9A. A reflecting surface of the beam splitter 9A has a predetermined light transmittance and a predetermined reflectance. The incident light beam is divided into two lights of a transmission light and a reflected light.
Between them, the transmission light passes through a detecting lens 16A and, after that, an astigmatism is given to the light through a cylindrical lens 17A. The light enters a PIN photodiode 18A and is used to detect a focusing error by an astigmatism method. A tracking error is detected by the PIN photodiode 18A by, for example, a push-pull method.
The reflected light which was reflected by the second beam splitter 9A passes through a half wave length plate 11A, so that a polarized light is rotated by 45.degree.. After that, the light is converted into a convergent light by a lens 12A and enters a polarization beam splitter 13A and is separated into two light beams whose polarized lights perpendicularly cross. The two light beams enter APD photodiodes 14A and 15A, respectively. A magneto-optic signal recorded on the magneto-optic disk 1A is reproduced by a detecting method, namely, a differential detecting method whereby a difference between detection signals of the APD photodiodes 14A and 15A is obtained by a subtracter such as a differential amplifier or the like (not shown).
According to the conventional optical head as mentioned above, however, as shown in FIG. 2, in the case where dust D or the like is deposited on the transparent substrate la having a refringence of the magneto-optic disk 1A, the magneto-optic signal fluctuates. Such a fluctuation changes depending on a size of dust, a position of dust D in the light beam entering onto the transparent substrate 1a, and further, a refringence value of the transparent substrate 1a. In the worst case, as shown in FIG. 3, a DC level of the magneto-optic signal changes like an S-shape. The above phenomenon will now be described hereinbelow with reference to the drawings.
Generally, a polycarbonate substrate (hereinafter, simply referred to as a PC substrate) which is cheap and has a good mass-productivity is used as a material of the transparent substrate of the magneto-optic disk. The PC substrate has a birefringence. FIG. 4 shows a refractive index ellipsoid (distribution of a refractive index due to the polarizing direction) 212 indicative of the birefringence of the PC substrate. The refractive index ellipsoid 212 of the PC substrate can be expressed as a negative uniaxial anisotropy medium in which the direction of an optic axis is set to the direction of a plate thickness and a refractive index Ne of an extraordinary light is smaller than a refractive index No of an ordinary light (accurately speaking, biaxial anisotropy medium in which a refractive index in the radial direction and a refractive index in the tangential direction in the plane of the disk are different and an optic axis is also slightly deviated from the direction of the plate thickness).
FIG. 5 schematically shows a refractive index in the incident direction of the light at an arbitrary position in the-light beam and a linearly polarized light of incidence in the case where a parallel light beam 100 of the optical head (FIGS. 1 and 2) in the above conventional technique is converged onto the recording surface 1b by the objective lens 8A and is again reflected and, after that, the light beam passes through the PC substrate or the like as a negative uniaxial anisotropy medium mentioned above. In the diagram, since the center light beam is the incident light beam from the direction of the optic axis of the negative uniaxial anisotropy medium, the refractive index has a circular distribution 300. The other light beams obliquely progress for the plate thickness direction of the transparent substrate 1a as shown in FIG. 2. Therefore, since the light beams become the incident light beams in the directions other than the optic axis direction of the negative uniaxial anisotropy medium, the refractive index has an elliptic distribution 301. As for an elliptic ratio (ratio of the major axis and the minor axis) of the ellipse, since the light beam (102) progresses in a manner such that as the light at the outer periphery of the light beam (102) has a larger inclination with respect to the optic axis of the negative uniaxial anisotropy medium, the elliptic ratio of the refractive index is large. Namely, the major axis of the ellipse shows the refractive index No of the negative uniaxial anisotropy medium shown in FIG. 4 and the minor axis of the ellipse of the light at the outer periphery decreases from the refractive index Ne in accordance with the outer peripheral position. A change amount in this case depends on an angle .alpha. of incidence (FIG. 2) to the transparent substrate 1a.
FIG. 6 schematically shows a polarization state in a certain radius in a reflected light beam 102 in the case where the parallel light beam 100 is converted into a convergent light 101 by the objective lens 8A of the optical head (FIGS. 1 and 2) in the above conventional technique, the convergent light 101 passes through the transparent substrate 1a which gives a refractive index distribution of FIG. 5 and is irradiated onto the recording surface 1b and, after that, the light is again reflected by the magneto-optic disk 1A and is converted into the parallel light as a reflected light beam 102 by the objective lens 8A. In the diagram, the polarized light corresponding to the radial direction or tangential direction is a linearly polarized light 102a (it is now assumed that there is no rotation of the plane of polarization due to a magneto-optic effect) in which the polarization state (FIG. 5) of the parallel light beam 100 entering the magneto-optic disk 1A is held. The polarized light in a region other than the radial direction or tangential direction, however, becomes an elliptically polarized light 102b. In the diagram, a phase difference (the direction of the major axis and the elliptic ratio) of the actual elliptically polarized light differs in accordance with the position of the incident light beam, namely, the incident angle .alpha. (FIG. 2) to the transparent substrate 1a or an angle .beta. with the polarizing direction of incidence.
When the reflected light beam 102 having a disturbance of the polarization state shown in FIG. 6, namely, a polarization distribution enters a differential detecting system (reflection optical system of the second beam splitter 9A in FIG. 1) and becomes a light beam 103 reflected by the polarization beam splitter 13A as an analyzer and a light beam 104 which was transmitted, their intensity distributions become different light/dark intensity distributions as shown in FIG. 7. As shown in the diagram, therefore, reduction amounts of the light amount due to an influence by the dust on the transparent substrate 1a differ depending on the light beams 103 and 104. Namely, when there is a dust A at positions in FIGS. 7A and 7B with respect to the light beam 103 since the light portion of the intensity of the light beam 103 is shielded (regions 103x and 103y in FIG. 7A) by the dust, the reduction of the light amount is large. On the other hand, since the dark portion of the intensity of the light beam 104 is shielded (regions 104x and 104y in FIG. 7B) by the dust, the reduction of the light amount is small. The reason why the light shielding portions by the dust exist at two positions in the light beam in the diagram is because one dust in the diagram exists at a position that is deviated from the center of the light beam. Namely, this is because the regions 103x and 104x where the light beam is shielded on the incident light beam side (before reaching the recording surface 1b) and the regions 103y and 104y where the light beam after it was reflected by the disk is shielded are caused. Therefore, when the position of the dust A in FIGS. 7A and 7B is moved by the rotation of the magneto-optic disk 1A and the dust A arrives at positions in FIGS. 8A and 8B, in a manner opposite to the case of FIGS. 7A and 7B, since the dark portion of the intensity of the light beam 103 is shielded (regions 103x' and 103y' in FIG. 8A) by the dust, the reduction of the light amount is small and, since the light portion of the intensity of the light beam 104 is shielded (regions 104x' and 104y' in FIG. 8B) by the dust, the reduction of the light amount is large. Therefore, after the light beams 103 and 104 entered the APD photodiodes 14A and 15A, respectively, a DC level (the DC level is generally optically set to the 0 level in case of differentially detecting) of the magneto-optic signal which is obtained by performing a subtraction by a subtracter is shown like an S-shape as shown in FIG. 3. Generally, as a signal process to demodulate a waveform of the magneto-optic signal into a data signal, as shown in FIG. 9, the magneto-optic signal is binarized (High level, Low level) by a predetermined slice level (level at which the signal is symmetrically sliced at the center of the signal), thereby converting into a rectangular wave pulse signal. When the DC level of the magneto-optic signal fluctuates due to the factors as mentioned above, a deviation occurs in the center of the magneto-optic signal for the slice level (the signal is asymmetrically sliced). There is, consequently, a subject such that the magneto-optic signal cannot be converted into the rectangular wave pulse which was correctly binarized and a signal detection error occurs.
In the foregoing conventional technique, therefore, a compensator or phase plate 10A is provided in the detection optical system (parallel light beam) of the optical head in order to compensate a birefringence occurring in the transparent substrate 1a.
The phase difference in the reflected light beam occurring in the transparent substrate 1a, however, is not a constant phase difference but has a distribution as shown in an example of FIG. 6. Therefore, a technical subject such that the refringence cannot be compensated by the technique in which the compensator or phase plate 10A which causes a predetermined phase difference into the parallel beam arriving at the detection optical system as in the above conventional technique still remains.